Semiconductor device having a die pad with rounded edges and its manufacturing method

ABSTRACT

A semiconductor device employs a lead frame including a die pad (24) and a plurality of leads (25) provided outside the die pad, and is manufactured by sealing the die pad and its periphery by a resin after the die pad is fitted with the semiconductor chip (11). The die pad (24) is formed separately from the main part of the lead frame provided with leads, and is rounded at an entire outermost edge thereof and includes a flat plate shape. This die pad can be either formed rounded with ceramic or resin, or formed in metal and given a rounded edge through honing.

FIELD OF THE INVENTION

The present invention relates to a semiconductor device and itsmanufacturing method, and particularly to a semiconductor device whichemploys a lead frame and is hermetically sealed by resin, and a methodof manufacturing such a semiconductor device.

DESCRIPTION OF THE RELATED ART

FIG. 1 is a side cross-sectional view showing an example of conventionalsemiconductor devices which employ lead frames and sealed by resin.

In FIG. 1, 11 represents a semiconductor chip, 12 a resin package, 13wires, 14 a die pad, 15 leads, 16 a support bar.

The semiconductor chip 11 is fixed on the die pad 14, which has a flatplate shape, and is connected to the leads 15 through the wires 13 bywire-bonding. The leads 15 make external connection with circuits formedonto the semiconductor chip 15.

The resin package 12 seals the semiconductor chip 11 and its periphery,and protects the semiconductor chip 11 and its periphery from theenvironment. The periphery includes the die pad 14, the wires 13 andthose parts of the leads 15 attached to the semiconductor 11.

A semiconductor device, as described above, is generally called aresin-sealed semiconductor device. Such a semiconductor device ismanufactured by using the die pad 14 and a lead frame having the leads15 disposed outside the die pad 14. The semiconductor chip 11 is mountedonto the die pad 14 (and the wires 13 are connected). Then, the resinpackage 12 is formed by means of resin molding.

The lead frame used in the manufacture practiced in the prior art isformed by cutting it from a metal plate by press working or etching in apattern such that the leads 15 and the support bar 16 are connected,after providing the support bar 16 in an arrangement such that it isplaced, as a support for the die pad 14, in alignment with the leads 15and extends from the die pad 14. Excess parts from that operation arecut off after the resin package 12 is formed.

Recently, shifts toward downsizing of semiconductor devices, and towardusing surface packaging, has brought about a serious problem in whicheither spontaneous moisture absorption of the resin package 2, orthermal stress at the time of the printed board packaging cause thedestructive crack on the resin package 12 to start at an edge of the diepad 14 as shown in FIG. 2A. This results from the fact that theoutermost edge of the die pad 14 is sharp.

This has been conventionally dealt with by chamfering the edge of thedie pad 14, on the lead frame, by press working. This, however, caneasily create a burr 18 during chamfering, making it impossible to fullyprevent a crack 7 from resulting due to this burr 18.

The present inventor attempted, as a simple means of performingchamfering of the die pad, an etching in which the entire lead frame issoaked in a chemical. The following problems arose, however, resultingin the lead frame not performing the function expected of it. Etchingdeveloped extensively on the leads and other narrow parts before thedesired chamfer of the die pad was obtained, rendering etched parts outof shape, and resulting in an irregular thickness of the lead frame, andin inaccuracy, when feeding the lead frame, at the time a semiconductorchip was mounted on it. An extremely high number of resin burrs wascreated in the resin molding process for forming the resin package.Following the resin package forming, the dimension of bending becameunstable in the lead bending.

Japanese Laid-Open Patent Application No. 1-251747 describes a processin which resist is applied to the die pad of the lead frame and the edgeof the die pad is chamfered by means of etching.

However, applying resist to the formed lead frame makes it difficult toensure high precision due to a deformation of the lead frame. Thus theprocess has been accompanied by the difficulty and inpracticability ofchamfering the edge even if it is etched.

SUMMARY OF THE INVENTION

Accordingly, a general object of the present invention is to provide anovel and useful semiconductor device, and a method of manufacturing it,in which the disadvantages of the aforementioned prior art areeliminated.

A more specific object of the present invention is to provide asemiconductor in which the occurrence of cracks in the resin package,starting at the edge of the die pad, is forestalled.

Another object of the present invention is to provide a semiconductordevice employing a lead frame including a die pad and a pluarality ofleads provided outside the die pad, and manufactured by sealing the diepad and its periphery by resin after the die pad is fitted with asemiconductor chip, wherein:

the die pad being formed separately from a main part of the lead framehaving leads, and rounded on an entire outermost edge of the die pad,and has a flat plate shape.

Another object of the present invention is to form a ceramic die padrounded.

Another object of the present invention is to form a resin die padrounded.

Another object of the present invention is to form the die pad withmetal and round it by honing.

Another object of the present invention is to create a radius ofcurvature greater than 0.03 mm when rounding the edge of the die pad.

Another object of the present invention is to provide a method ofmanufacturing a semiconductor comprising the steps of forming a die padwith its outermost edge rounded,

forming a main part of a lead frame including interconnected leads, andsupport bar for supporting the die pad,

joining the support bar with the die pad,

mounting a semiconductor chip on the die pad with which the support baris joined,

wire-bonding the semiconductor chip with the pluarality of leads; and

sealing, by resin, the semiconductor chip onto the die pad and thewire-bonded leads.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side cross-sectional view of an example semiconductor devicein the prior art;

FIG. 2A illustrates a problem with the prior art;

FIG. 2B illustrates a problem with the prior art;

FIG. 3 is a side cross-sectional view of an example embodiment of thesemiconductor device of the present invention;

FIG. 4A is a plan view of the lead frame;

FIG. 4B is a partially enlarged side cross-sectional view of the leadframe; and

FIG. 5 is a diagram illustrating a manufacturing process of thesemiconductor device of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 3 is a side cross-sectional view of an embodiment of the presentinvention. FIGS. 4A and 4B are, respectively, a plan view of a leadframe, and a partially enlarged side cross-sectional view of the leadframe. Parts that are the same from figure to figure have the samereference numerals.

Referring to FIG. 3, the embodiment of the semiconductor deviceillustrated in the figure is manufactured by employing a die pad and alead frame mentioned in the following as in the case of the prior artillustrated in FIG. 1. The semiconductor chip 11, the resin package 12,and the wires 13 are the same as those in the prior art while the diepad 14 of the prior art is replaced with a die pad 24, and is separatedfrom leads 25. In the figure, 26 represents a support bar.

The die pad 24 comprises sintered ceramic formed by molding AlN(aluminum nitride), for example, into a flat plate shape, as done in theprior art, and producing a rounded edge 24a on the entire outermostedge. AlN, a material of the die pad 24, has, as compared with the resinpackage 12, a small coefficient of linear expansion, a small thermalstress, good contact with the resin package 24, and a great coefficientof thermal conductivity, attributes that make it most suitable as amaterial for the die pad 24.

The die pad 24 can be formed not only with a ceramic, such as AlN, butalso with such metals as Fe containing 42% nickel or Ni alloy (42alloy), or a resin. When using metal, the metal should be fully honed,with alumina powder (barrel polishing) for example, in order to producethe rounded edge 24a. When using resin the chamfer 24a is produced atthe time of forming, as in the case of ceramic. Ceramic and resin havean advantage in that they do not require any additional process to formthe rounded edge 24a.

In this invention, the rounded edge 24a is very easily formed becausethe die pad 24 is produced as an independent part.

The radius of curvature of the rounded edge 24a should be about 0.03 mm.This rounded edge 24a eliminates the sharpness of the outermost edge ofthe die pad 24, thus preventing the crack 7, illustrated in FIG. 2A,from occurring in the resin package 2.

The following data obtained in experiments, including 100 samples foreach radius of curvature of the rounded edge concern the relationshipbetween the radius of curvature of the rounded edge 24a and theoccurrence of the crack in the semiconductor device.

    ______________________________________                                        Radius of curvature                                                           of the rounded edge                                                                          Occurrence of the crack                                        ______________________________________                                        0.02 mm        18%                                                            0.03 mm        0%                                                             0.04 mm        0%                                                             0.05 mm        0%                                                             ______________________________________                                    

It is evident from this that by ensuring that the radius of curvature ofthe rounded edge 24a is greater than or equal to 0.03 mm it is possibleto sufficiently prevent the occurrence of the crack 17. In principle, itis preferable that this radius of curvature be large.

A description is given below of an example of an embodiment of the leadframe used in manufacturing the aforementioned example semiconductordevice.

This lead frame has the leads 25 arranged at the outside of the die pad24, and with the support 26 set along the leads 25, for support of thedie pad 24. This assembly of the leads 25 and the support bar 26 is cutfrom a metal plate by means of press working or etching in a patternthat makes them interconnected. The main part of the lead frame, whichis produced separately from the independently produced die pad 24, isintegrated with the die pad 24 through conjunction with the support bar26. Accordingly, the choice of material for the die pad 24 is notlimited to metal used in the main part of the lead frame, but caninclude any metal, ceramic or resin. Unneeded excess parts are cut afterthe package 12 is formed, as in the case of the example prior artmanufacture shown in FIG. 1.

The most characteristic point of this lead frame is that the entireoutermost edge of the die pad 24 is provided with the rounded edge 24a,as shown in the partially enlarged side cross-sectional view of FIG. 4B.An explanation has already been given of the radius of curvature of therounded edge 24a, and its utility.

Joining of the die pad 24 and the support bar 26 can be performed bylaser welding in the case of ceramic and resin, electric welding in thecase of metal, an adhesive in the case of resin, for example.

The thickness of the die pad 24 may be chosen on the basis of the aboveknowledge, and a thickness similar to that of the main part of the leadframe, for example, is preferable.

In FIG. 4A and FIG. 4B, the support bar 26 is joined to the top of diepad 24; it can, alternatively, be joined to its bottom.

FIG. 5 is a diagram illustrating the process of the semiconductor devicemanufacture practiced in the present invention.

Referring to FIG. 5, at step 40 the die pad 24 is formed with therounded edge 24a formed on it. As mentioned previously, ceramic andresin require only one process in order to form the die pad 24, whereasmetal requires an additional process of polishing.

Cutting of the main part of the lead frame is performed at step 41 atthe same time as the above mentioned step 40 takes place. Plating of themain part of the lead frame is done at step 42.

The lead frame is completed at step 43, by joining the die pad 24 withthe support bar 26 in the main part of the lead frame. The semiconductorchip 11 is mounted onto the die pad 24 at step 44, and at step 45 thewires 13 are wire-bonded between the semiconductor chip 11 and the leads15. At step 46 the resin package 12 is molded, and at step 47 thepackage is formed by cutting excess parts off the lead frame, thuscompleting the semiconductor device.

Thus, the occurrence of the crack 17 is caused by the sharpness of theoutermost edge of the die pad (shown as 4 in the prior art example).When the entire outermost edge of the die pad is rounded, occurrence ofthe aforementioned crack is prevented because there is no area having asharp edge. Ensuring that the radius of curvature of the chamfer isgreater than or equal to 0.03 mm satisfactorily prevents the occurrenceof the crack, as mentioned in the following.

Also, the aforementioned lead frame can be easily chamfered because thedie pad is an independent part; this also leads to a variety in thechoice of materials for the die pad.

The present invention prevents cracks in the resin package from startingat the edge of the die pad because the outermost edge of the die pad ischamfered. This invention makes rounding of the outermost edge of thedie pad possible, by producing the die pad separately from the main partof the lead frame.

We claim:
 1. A semiconductor device manufactured employing a lead frameincluding a die pad and a plurality of leads provided outside said diepad, and manufactured by sealing of said die pad and its periphery byresin after said die pad is fitted with a semiconductor chip, said diepad being formed separately from a main part of the lead frame havingleads and having a flat plate shape, wherein said die pad has a roundedoutermost edge having a radius of curvature greater than or equal to0.03 mm.
 2. A semiconductor device as claimed in claim 1, wherein saiddie pad is made of a ceramic and said outermost edge is rounded duringforming of said die pad.
 3. A semiconductor device as claimed in claim1, wherein said die pad is made of a resin and said outermost edge isrounded during forming of said die pad.
 4. A semiconductor device asclaimed in claim 1, wherein said die pad is made of a metal and saidoutermost edge is rounded by honing.
 5. A method of manufacturing asemiconductor comprising the steps of:forming a die pad with a roundedoutermost edge having a radius of curvature greater than or equal to0.03 mm, forming a main part of a lead frame including interconnectedleads and a support bar for supporting said die pad, joining saidsupport bar with said die pad, mounting a semiconductor chip on the diepad with which said support bar is joined, wire-bonding saidsemiconductor chip with the plurality of leads, and sealing, by resin,the semiconductor chip onto the said die pad and the wire-bonded leads.6. A semiconductor device as claimed in claim 2, wherein the area ofsaid die pad is less than an area defined by tip ends of the pluralityof leads.